During forming of metals using various bulk deformation processes, the magnitude of force required to perform deformation is a significant factor in terms of the manufacturing of parts. Generally, as the force necessary to deform a given material increases, larger equipment must be utilized, stronger tools and dies are required, tool and die wear increase, and more energy is consumed in the process. All of these factors increase the manufacturing cost of a given component. Therefore, any method or apparatus that would decrease the force required for deformation and/or increase the amount of deformation that can be achieved without fracture would have a significant impact on many manufacturing processes.
Presently, deformation forces are reduced and elongation is increased by working metals at elevated temperatures. However, significant drawbacks to deforming materials at elevated temperatures exist, such as increased tool and die adhesion, decreased die strength, decreased lubricant effectiveness, consumption of materials for heating (which raises energy cost) and the need for additional equipment to be purchased.
One possible method of deforming metallic materials without using such elevated temperatures is to apply an electric current to the workpiece during deformation. In 1969, Troitskii found that electric current pulses reduce the flow stress in metal (Troitskii, O. A., 1969, zhurnal eksperimental'noi teoreticheskoi kiziki/akademi'i'a nauk sssr—pis'ma v zhurnal. eksperimental' i teoretiheskoi fiziki, 10, pp. 18). In addition, work by Xu et al. has shown that continuous current flow can increase the recrystallization rate and grain size in certain materials (Xu, Z. S., Z. H. Lai, Y. X. Chen, 1988, “Effect of Electric Current on the Recrystallization Behavior of Cold Worked Alpha-Ti”, Scripta Metallurgica, 22, pp. 187-190). Similarly, works by Chen et al. have linked electrical flow to the formation and growth of intermetallic compounds (Chen, S. W., C. M. Chen, W. C. Liu, Journal Electron Materials, 27, 1998, pp. 1193; Chen, S. W., C. M. Chen, W. C. Liu, Journal Electron Materials, 28, 1999, pp. 902).
Using pulses of electrical current instead of continuous flow, Conrad reported in several publications that very short-duration high-density electrical pulses affect the plasticity and phase transformations of metals and ceramics (Conrad, H., 2000, “Electroplasticity in Metals and Ceramics”, Mat. Sci. & Engr., A287, pp. 276-287; Conrad, H., 2000, “Effects of Electric Current on Solid State Phase Transformations in Metals”, Mat. Sci. & Engr. A287, pp. 227-237; Conrad, H., 2002, “Thermally Activated Plastic Flow of Metals and Ceramics with an Electric Field or Current”, Mat. Sci. & Engr. A322, pp. 100-107). More recently, Andrawes et al. has shown that high levels of DC current flow can significantly alter the stress-strain behavior of 6061 aluminum (Andrawes, J. S., Kronenberger, T. J., Roth, J. T., and Warley, R. L., “Effects of DC current on the mechanical behavior of AlMgISiCu,” A Taylor & Francis Journal: Materials and Manufacturing Processes, Vol. 22, No. 1, pp. 91-101, 2007). Complementing this work, Heigel et al. reports the effects of DC current flow on 6061 aluminum at a microstructural level and showed that the electrical effects could not be explained by microstructure changes alone (Heigel, J. C., Andrawes, J. S., Roth, J. T., Hoque, M. E., and Ford, R. M., “Viability of electrically treating 6061 T6511 aluminum for use in manufacturing processes,” Trans of N Amer Mfg Research Inst, NAMRI/SME, V 33, pp. 145-152).
The effects of DC current on the tensile mechanical properties of a variety of metals have been investigated by Ross et al. and Perkins et al. (Ross, C. D., Irvin, D. B., and Roth, J. T., “Manufacturing aspects relating to the effects of DC current on the tensile properties of metals,” Transactions of the American Society of Mechanical Engineers, Journal of Engineering Materials and Technology, Vol. 29, pp. 342-347, 2007; Perkins, T. A., Kronenberger, T. J., and Roth, J. T., “Metallic forging using electrical flow as an alternative to warm/hot working,” Transactions of the American Society of Mechanical Engineers, Journal of Manufacturing Science and Engineering, vol. 129, issue 1, pp. 84-94, 2007). The work by Perkins et al. investigated the effects of currents on metals undergoing an upsetting process. Both of these previous studies included initial investigations concerning the effect of an applied electrical current on the mechanical behavior of numerous materials including alloys of copper, aluminum, iron and titanium. These publications have provided a strong indication that an electrical current, applied during deformation, lowers the force and energy required to perform bulk deformations, as well as improves the workable range of metallic materials. Recently, work by Ross et al. studied the electrical effects on 6Al-4V titanium during both compression and tension test (Ross, C. D., Kronenberger, T. J., and Roth, J. T., “Effect of DC Current on the Formability of 6AL-4V Titanium,” 2006 American Society of Mechanical Engineers—International Manufacturing Science & Engineering Conference, MSEC 2006-21028, 11 pp., 2006).
Electrical current is the flow of electrons through a material. The electrical current meets resistance at the many defects found within materials, such as: cracks, voids, grain boundaries, dislocations, stacking faults and impurity atoms. This resistance, termed “electrical resistance”, is widely known and extensively measured. The greater the spacing that exists between defects, the less resistance there is to optimal electron motion. Conversely, the less spacing between these defects, the greater the electrical resistance of the material.
During loading, material deformation occurs by the movement of dislocations within the material. Dislocations are line defects which can be formed during solidification, plastic deformation, or be present due to the presence of impurity atoms or grain boundaries. Dislocation motion is the motion of these line defects through the material's lattice structure causing plastic deformation.
Dislocations meet resistance at many of the same places as electrical current, such as: cracks, voids, grain boundaries, dislocations, stacking faults and impurity atoms. Under an applied load, dislocations normally move past these resistance areas through one of three mechanisms: cross-slip, bowing or climbing. As dislocation motion is deterred due to localized points of resistance, the material requires more force to continue additional deformation. Therefore, if dislocation motion can be aided through the material, less force is required for subsequent deformation. Theoretically, this will also cause the material's ductility to be subsequently increased.